Support deletion of test cases

[chiiya]

We should add support for the deletion of test cases. As an example: sometimes query parameters might be incompatible and the server might return a client error if the request contains both of the parameters. It's not possible currently to specify this case in the API specification, and thus comet will generate a test case covering this scenario. The user should be able to delete the test case, and it should not be re-generated the next time comet is run. A proposal would be:

Have a .history or .lock file or something similar where we store a dictionary of all test case names, with an associated boolean value. The procedure when creating a test case in the decorator would be:

1. Check if the test case has an entry in the lock file.
2. If it does, and its value is false, do not create the test case.
3. If it does not, create the test case.
4. If it does, and its value is true, check the relevant file generated last time, and check for the existance of that test case. If it exists, go ahead and create the test case. If it does not exist, update the lock file (change corresponding value to false) and do not create the test case.